Maltulose (4-O-.alpha.-D-glucopyranosyl-D-fructose) is not a product widely found in the natural world. It is found however in honey where it is synthesised by transglucosylation reaction during the hydrolysis of sucrose by D-glucosidases and also in yeasts, where it is produced by an equivalent enzymatic mechanism. Maltulose is also detected in waxy maize starch hydrolysates, and in the .alpha.-amylolysis hydrolysates of the glycogen in the liver of rats.
Maltulose has significant sweetening power, equivalent to that of maltitol, less than that of saccharose but greater than that of dextrose and maltose. Maltulose is used in the form of syrups in the food and pharmaceutical industries, and as a source material for various industrial syntheses.
The industrial preparation of a maltulose syrup is based on the isomerisation of a maltose syrup in an alkaline medium (HOUGH et al., 1953, J. Chem. Soc., Part II, 2005; ROBERTS et al, Biochem. J., 1952, 51, XVII). To illustrate such a process, the works of HODGE et al (Cereal Science Today, 1972, 17, 180) are worthy of mention, since they describe the preparation of a maltulose syrup by chemical isomerisation of a maltose syrup in the presence of sodium aluminate with conversion yields of about 95%.
Processes allowing maltulose syrups to be obtained by isomerisation of maltose enriched starch hydrolysates are also described in FR 2 055 645 and U.S. Pat. Reissue No. 30 280.
In terms of obtaining pure crystals of maltulose from these syrups, two processes are known, namely:
the one described by HOUGH et al. in J. Chem, Soc., 1953, Part II, 2005, with regard to the crystallisation of the maltulose monohydrate in the mixture dioxan-methanol, which produces a compound with a melting point (mp) of between 113 and 115.degree. C.; PA1 the one described by PFEFFER and HICKS in Carb. Research., 1983, 111, 181, describing the crystallisation of maltulose monohydrate in acetone. The crystals, washed in acetone and dried in the air, have an mp of 116.5 to 118.degree. C. PA1 the preparation of an aqueous maltulose solution of a strength above 65% by weight, PA1 the concentration of the aqueous maltulose solution to a dry matter ratio of more than 50% by weight and at a temperature such that the degree of maltulose supersaturation is less than 1, PA1 the cooling of said concentrated solution so as to take the degree of maltulose supersaturation to a value above 1, PA1 the crystallisation of the maltulose in said supersaturated solution, by cooling it at a controlled speed and by stirring, and PA1 the separation, recuperation and drying of the maltulose monohydrate crystals thus obtained.
It seems therefore clear from the prior art that maltulose crystallisation processes require the use of toxic and flammable organic solvents (dioxan-methanol, acetone) to obtain maltulose crystals. But the use of such solvents involves a degree of hazard for the people handling them, and indeed for the environment.
Above all, these processes demand a thorough purification of the crystallised maltulose so as to eliminate all traces of organic solvents. Indeed, these solvent traces would undoubtedly present toxicity within the scope of the food and pharmaceutical applications of the maltulose.
The previously described processes are not therefore satisfactory since they suffer the drawback of not being able to offer a straightforward method of producing maltulose crystals without traces of toxic compounds resulting from the crystallisation stage. Consequently, the crystallised maltulose obtained according to known processes cannot be directly used in pharmaceutical applications, for example for the manufacture of powders or tablets.
The object of the invention is thus to remedy this situation, and to propose a process which meets the various constraints of practice better than those which already exist.
Indeed, the applicants have managed to perfect an industrial process enabling crystallised maltulose to be obtained from an aqueous solution, and consequently free from all toxic compounds.
In doing this, the applicants have overcome a technical prejudice against the use of such a process using water as a solvent to crystallise the maltulose, since it has never been suggested that maltulose may be crystallised directly and easily in water and still less that the crystals thus formed can be isolated from it.
On the contrary, because of the high solubility of maltulose in water, specialists in this field concluded that the crystallisation of maltulose from an aqueous solution was difficult to achieve industrially. Thus, in the HAYASHIBARA patent (FR 2 055 645) it is recalled that maltulose is very soluble in water and it is additionally stated that maltulose syrups are difficult to crystallise, even in very concentrated solutions, at the usual temperature. In the conditions described in this patent, the maltulose, when it crystallises, does so in the form of particles so fine and so poorly formed that they cannot in any case be separated out from the syrup.
This crystallisation in fact presents a certain number of technical difficulties, which the applicants have resolved after extensive research.